@article{DAN_1972_206_2_a6,
     author = {V. M. Petkov},
     title = {Necessary conditions for correctness of the {Cauchy} problem for nonstrictly hyperbolic equations},
     journal = {Doklady Akademii Nauk},
     pages = {287--290},
     publisher = {mathdoc},
     volume = {206},
     number = {2},
     year = {1972},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1972_206_2_a6/}
}